Four bells ring at intervals of 30 minutes, 1 hour, 1 hour 30 min and 1 hour 45 min respectively. All the bells ring simultaneously at 12 noon. They will ring again simultaneously at:

6 am 3 am 9 pm 9 am

9 am

30 min, 60 min, 90 min, 105 min LCM ⇒ 1260 min Convert into hour ⇒  \(\frac{1260}{60}\) ⇒ 21 hour All will ring again simultaneously on next morning at ⇒ 12:00 + 21 hour = 9:00AM